The present invention relates to a method for the preparation of a rare earth phosphate of low overstoichiometric phosphorus content or, more particularly, to a method for the preparation of a rare earth phosphate of which the content of phosphorus in excess of stoichiometry, i.e. equimolar contents of phosphorus and the rare earth element, can be greatly decreased in a precipitation reaction by mixing an aqueous solution of a rare earth salt and an aqueous phosphoric acid solution.
As is well known, several kinds of mixed rare earth phosphates are useful as a base material in the preparation of luminescent phosphors for cathode ray tubes (see, for example, U.S. Pat. No. 3,507,804). A composite phosphate of lithium, neodymium and ytterbium is useful as a base material of infrared-emitting phosphors (Japanese Patent Kokai 53-60888). Lanthanum orthophosphate of the monoclinic crystalline system activated with cerium and terbium is used as a base material of phosphors in three-band fluorescent lamps (U.S. Pat. No. 3,634,282 and Japanese Patent Kokai 54-56086).
As is readily understood, rare earth phosphates in general can be prepared by utilizing the method of precipitation in which an aqueous solution of phosphoric acid and an aqueous solution of a water-soluble rare earth salt are blended together so that the rare earth phosphate is formed in the form of precipitates which are recovered from the aqueous precipitation medium and dried and calcined.
It is known that the content of phosphorus in the rare earth phosphate obtained by the above mentioned precipitation method is usually in substantial excess over stoichiometry. When a phosphor for cathode ray tubes or fluorescent lamps is prepared from such a rare earth phosphate of overstoichiometric phosphorus content and the phosphor is used in the form of an aqueous slurry for coating, the excess of phosphorus in the phosphor is gradually leached out in the slurry to affect the stability of the organic binder usually contained in the slurry or hence the stability of the phosphor slurry eventually leading to gelation of the slurry. Further, the excess of phosphorus in the phosphor gradually reacts with the mercury in the fluorescent lamp to cause a decrease in the luminescent brightness of the phosphor. Incidentally, a deficiency in the content of phosphorus below stoichiometry is also undesirable in a rare earth phosphate used as a base material of phosphors because of the greatly decreased luminescent brightness of the phosphor prepared therefrom.
As a remedy for the above mentioned disadvantages due to the overstoichiometric phosphorus content in the rare earth phosphate, an improved method is proposed in Japanese Patent Kokai 5-171143 in which the rare earth phosphate containing an excess of phosphorus as prepared by the conventional method of precipitation and calcination is blended with an oxide of the same rare earth element or elements in combination in an amount equivalent to the overstoichiometric amount of phosphorus and the blend is subjected to a second calcination treatment so as to convert the excess of phosphorus into the phosphate. This method, however, cannot be fully effective in reducing the overstoichiometric phosphorus content in the rare earth phosphate presumably because mixing of two kinds of powders of, one, the rare earth phosphate and, the other, the rare earth oxide can never be intimate enough as an inherency in solid-solid mixing even by prolongedly continuing mixing.